Polyploidization is involved in a variety of biological processes. It is one of the principal mechanisms of new species formation since it provides material for gene diversification and subsequent selection. Multiple cases of polyploidization were registered in different branches of the evolutionary tree of eukaryotes. Besides its role in evolution, polyploidization affects gene expression in living cells: pathological genome duplications often happen in cancer cells. The mechanisms and consequences of polyploidization are being studied extensively. However, quantitative determination of the polyploidization rate is challenging due to its low frequency and the absence of selective genetic markers that would phenotypically distinguish between haploids and polyploids. Our work describes a robust and straightforward method for discriminating haploid and polyploid states in the yeast Saccharomyces cerevisiae, a model organism for studying different aspects of polyploidization. The measurement of polyploidization rate showed that in yeast cells this process is mainly caused by autodiploidization rather than mating-type switching followed by hybridization.